Levothyroxine

Synthroid is prescription replacement therapy for congenital or acquired primary, secondary or tertiary hypothyroidism in adults and children, including neonates. It also supports surgery and radioiodine treatment of thyrotropin-dependent, well-differentiated thyroid cancer.

It is not indicated for benign thyroid-nodule or nontoxic diffuse-goiter suppression in iodine-sufficient patients, or for the recovery phase of subacute thyroiditis. Do not prescribe for weight loss in a euthyroid patient.

The linked ready-to-use injection is labeled for adult myxedema coma. Oral-to-IV equivalence is not established; specialist emergency care and the exact IV label are required.

Doses above 200 mcg/day are seldom needed. A poor response above 300 mcg/day warrants assessment of adherence, absorption and interactions. A dose may take 4–6 weeks to reach its peak effect.

Adjust to laboratory results and development; usual pediatric titration is every 2 weeks. Infants at cardiac-failure risk start lower and increase every 4–6 weeks. For hyperactivity risk, the label starts at one-quarter of full replacement and increases by one-quarter weekly.

Take once daily on an empty stomach 30–60 minutes before breakfast. Separate absorption-interfering medicines by at least 4 hours; see the interaction card for resin-specific timing. For a child unable to swallow, freshly crush a tablet into 5–10 mL water, give immediately and ensure the full dose is swallowed. Do not store the mixture or mix into soy-based formula.

Check TSH and free T4 promptly when pregnancy is confirmed. For pre-existing primary disease with TSH above the trimester range, the label uses 12.5–25 mcg/day increases and TSH testing every 4 weeks until stable. For newly diagnosed disease, its starting doses are 1.6 mcg/kg/day if TSH ≥10 mIU/L and 1.0 mcg/kg/day if TSH <10 mIU/L. Return to the pre-pregnancy dose after delivery and check TSH at 4–8 weeks postpartum; individualize further changes.

The reviewed oral label supplies no fixed renal or hepatic dose-reduction formula. Dose to the disease-specific thyroid tests and clinical response; comorbid illness and altered hormone binding can complicate interpretation.

This is the Fresenius ready-to-use solution label, not a reconstitution instruction for powder products. Do not infer an exact oral-to-IV conversion.

Uncorrected adrenal insufficiency is the reviewed label’s contraindication. Hypersensitivity reactions to inactive ingredients have been reported; select and evaluate formulations carefully after suspected allergy.

Thyroid hormone must not be used for obesity or weight loss. Doses exceeding physiological needs can cause life-threatening toxicity, particularly with sympathomimetic appetite suppressants.

Overdose-like hyperthyroid effects include tremor, heat intolerance, insomnia, sweating, diarrhea, palpitations and arrhythmias. Serious cardiac events, seizures and pediatric intracranial hypertension or growth-plate complications are reported. Excipient allergy can cause rash, wheezing or angioedema. Symptoms after an overdose may be delayed for days; seek urgent assessment and contact U.S. Poison Control at 1-800-222-1222.

• Calcium, iron and phosphate binders: separate by at least 4 hours. Bile-acid sequestrants and ion-exchange resins: take Synthroid at least 4 hours before them or monitor TSH as specified by the label.
• Antacids, sucralfate and acid suppression can reduce absorption; review timing and monitor thyroid response. Orlistat also requires thyroid monitoring.
• Soy, dietary fiber, walnuts and grapefruit can alter absorption; maintain a consistent routine and reassess after dietary changes.

• Oral anticoagulants: thyroid correction can increase anticoagulant response; monitor coagulation and adjust treatment. Digoxin effects or levels can fall as euthyroidism is restored.
• Insulin and glucose-lowering drugs: requirements can rise; monitor glucose.
• Tricyclic/tetracyclic antidepressants, sympathomimetics and ketamine: cardiac or stimulatory toxicity can increase; monitor heart rate, pressure and symptoms. Sertraline may increase replacement requirements.
• Rifampin, phenobarbital and tyrosine-kinase inhibitors can change requirements. Amiodarone and high-dose glucocorticoids or propranolol alter T4-to-T3 conversion. Estrogens, androgens, phenytoin and other binding modifiers can alter tests; interpret free hormone and clinical status.

Biotin can distort thyroid immunoassays. Stop biotin-containing supplements for at least 2 days before TSH or T4 testing, as directed by the label, and disclose their use to the laboratory and clinician.

Untreated maternal hypothyroidism poses maternal and fetal risks. Available clinical experience with replacement doses has not shown increased major birth defects or miscarriage. Continue treatment and promptly treat newly diagnosed disease; requirements commonly increase. Use trimester-specific TSH targets in primary disease and the pregnancy monitoring schedule above.

T4 is a normal milk constituent. Limited data on replacement therapy have not shown adverse infant effects. Adequate maternal treatment can improve low milk production caused by hypothyroidism. Continue individualized postpartum thyroid testing; some patients with Hashimoto disease need more than their pre-pregnancy dose.

Treat congenital disease promptly to protect growth and neurodevelopment. Follow both thyroid tests and growth/bone maturation; monitor infants for cardiac overload or arrhythmias during the first 2 weeks. Older adults need a reduced starting dose and careful cardiac assessment.

Levothyroxine has the physiological effects of endogenous T4. Peripheral deiodination supplies much of circulating T3; thyroid-hormone nuclear receptors regulate gene transcription and protein synthesis.

Oral absorption

Approximately 40–80%; fasting improves absorption, while food, medicines and malabsorption can reduce it.

Distribution / metabolism

More than 99% protein bound; deiodination plus glucuronide/sulfate conjugation.

Half-life / elimination

T4 half-life about 6–7 days when euthyroid, longer in hypothyroidism; renal and fecal elimination.

• Primary hypothyroidism in adults: measure TSH 6–8 weeks after a dose change; once stable, reassess every 6–12 months and with clinical changes.
• Central hypothyroidism: TSH is unreliable for dose adequacy. Follow free T4, aiming for the upper half of its normal range with clinical euthyroidism.
• Children: check TSH and total/free T4 at 2 and 4 weeks after initiation, 2 weeks after a dose change, then every 3–12 months once stable until growth completes; assess development and bone maturation.
• Assess pulse, cardiac symptoms, glucose and relevant anticoagulation; investigate adherence, timing and absorption before escalating an unexpectedly high dose.

• Confirm the tablet strength in micrograms and follow one consistent dosing routine. Improvement takes weeks; replacement is often lifelong.
• Tell the clinician about pregnancy, breastfeeding, supplements, heart disease and adrenal/pituitary disease. Stop biotin before tests as directed.
• Report palpitations, chest pain, shortness of breath, marked tremor or allergic symptoms promptly. Do not use thyroid hormone as a weight-loss medicine.

Dosage form / strength

Oral tablet · 50 mcg

Label packager

AbbVie Inc.

Appearance / imprint

White, round · SYNTHROID and 50

Example package NDC

0074-4552-90 · Bottle of 90

Synthroid tablets

25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 and 300 mcg.

Linked ready-to-use IV solution

100, 200 or 500 mcg per 5 mL single-dose vial.

Other products

Oral liquid, capsule and injection products have separate labeling; do not apply tablet preparation instructions to them.

Store Synthroid at 20–25°C; excursions to 15–30°C are permitted. Protect from light and moisture. Follow the exact package instructions for other formulations.